A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation source for producing EUV radiation may include an excitation beam, such as a laser, for exciting a fuel to provide the plasma, and an enclosure for containing the plasma. The plasma may be created, for example, by directing a laser beam (i.e., an excitation beam providing radiation for initiation of the plasma) at a fuel, such as particles (i.e., droplets) of a suitable fuel material (e.g., tin, which is currently thought to be the most promising and thus likely choice of fuel for EUV radiation sources), or at a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector (sometimes referred to as a near normal incidence radiation collector), which receives the radiation from the plasma and focuses the radiation into a beam. The radiation collector may have any other suitable form. The radiation source may include an enclosure or chamber arranged to provide a vacuum environment to support the plasma, and typically the radiation collector will be located within the enclosure. Such a radiation system is typically termed a laser produced plasma (LPP) source, when a laser is used to provide the beam of excitation radiation. In an alternative system, which may also employ the use of a laser, radiation may be generated by a plasma formed by the use of an electrical discharge—a discharge produced plasma (DPP) source.
The present application is concerned with radiation sources, and methods of generation of radiation, particularly EUV radiation for use in lithography, where the radiation, is produced from a plasma generated by excitation of a fuel particles, typically molten metal fuel droplets, by means of an excitation beam, which may typically be a laser beam, such as an infra-red laser beam. Such radiation sources include LPP radiation sources and for the sake of brevity, such a source is referred to hereinafter as an LPP radiation source, although it will be understood that the excitation beam is not necessarily limited to being a laser beam, and any other suitable excitation beam (or combination of excitation beams) may be employed.
In an LPP radiation source a stream of fuel particles is typically arranged to travel in a trajectory passing through or near a focal point of the excitation beam. As the particles cross the path of the excitation beam near the focal point, the fuel particles are heated up to an extremely high temperature by the focused beam and a plasma is formed of high energy ions of the fuel material, and electrons. In the plasma the atoms of the fuel material are stripped of their outer electrons. As the electrons return to the ions, photons of EUV radiation are emitted.
Typically, the stream of fuel particles may be generated as a stream of molten fuel droplets by an initially continuous jet or stream of fuel, as molten liquid, being decomposed into small droplets to form the stream of fuel particles. As used herein the term “particle” means either solid or preferably liquid fuel as small, separate portions of fuel. Fuel droplet generators may comprise a nozzle through which molten fuel is driven under pressure to be injected from the nozzle as a stream of droplets. The natural break-up of a stream of liquid issuing from a nozzle is known as Rayleigh break-up. The Rayleigh frequency, which corresponds to the rate of droplet production of the nozzle is related to the mean velocity of the fuel at the nozzle and the diameter of the nozzle:
      f    Rayleigh    =            mean      ⁢                          ⁢      velocity              4.5      ⁢                          ⁢      nozzle      ⁢                          ⁢      diameter      
Although Rayleigh break-up of a stream of fuel may occur without excitation, a vibrator such as a piezoelectric actuator may be used to control the Rayleigh break-up by modulating or oscillating the pressure of molten fuel at the nozzle. Modulating the pressure inside the nozzle may modulate the exit velocity of the liquid fuel from the nozzle, and cause the stream of liquid fuel to break-up into droplets in a controlled manner directly after leaving the nozzle.
If the frequency of oscillation applied by a vibrator is sufficiently close to the Rayleigh frequency of the nozzle, droplets of fuel are formed, the droplets being separated by a distance which is determined by the mean exit velocity from the fuel nozzle and by the oscillation frequency applied by the vibrator. If the oscillation frequency applied by the vibrator is substantially lower than the Rayleigh frequency, then instead of a periodic stream of small fuel droplets being formed, aligned groups of fuel small droplets may be generated. A given aligned group of fuel may include a group of small droplets travelling at a relatively high speed and a group of small droplets travelling at a relatively low speed (the speeds being relative to the average speed of the stream of fuel exiting the nozzle). These aligned groups may coalesce together to form single larger fuel droplets. In this way a periodic stream of fuel droplets may be generated by applying an oscillation frequency to the vibrator which is significantly lower than the Rayleigh frequency. The spacing between the droplets is still governed by mean exit velocity and the oscillation frequency: the spacing between the droplets increases with decreasing oscillation frequency.
Typically, a piezoelectric transducer may be used as a vibrator to apply oscillation to a nozzle such as a glass capillary. The piezoelectric transducer may be driven by a waveform generator with a signal that may contain a high frequency to break up the jet and a low frequency to control the coalescence behavior. Molten fuel may be stored in a heated reservoir vessel and forced to flow towards the nozzle through a filter. The flow rate may be maintained by a gas pressure over the molten fluid fuel in the reservoir vessel.
In order to keep the collector optics of the radiation source clean from condensing fuel, gas, such as hydrogen gas (which may optionally contain hydrogen radicals), may be directed as a flow to transport contaminating fuel vapor and debris particles away from the radiation collector optics. The amount of fuel used in a radiation source may be selected as a compromise between the desired radiation power generated and the contamination of the inside of the radiation source enclosure, particularly the radiation collector optics.
For a typical arrangement, the fuel particles may be roughly spherical molten droplets (e.g., of tin), with a diameter about 30 μm, with the waist of the focused excitation source (usually an infra-red laser beam) being 60 to 450 μm in diameter at its focal point. Droplets are typically generated at frequencies between 40 to 80 kHz and directed towards the focused region of the excitation beam with velocities typically from 40 to 120 m/s.